We are actively studying the regulation of intracellular enzyme level in relation to protein turnover and aging. We have previously characterized the degradation of Escherichia coli glutamine synthetase (GS) and have presented evidence for a two-step process involving inactivation followed by proteolysis. Furthermore, we have characterized several enzymic mixed-function oxidation systems which catalyze inactivation of GS. The generality of the oxidative inactivation reaction is supported by the demonstration that several key metabolic enzymes other than GS are inactivated in a qualitatively similar manner by at least two enzymic mixed-function oxidation (MFO) systems, namely, microbial NADH-oxidase and the cytochrome P-450 system. We are currently examining the possible physiological role of MFO catalyzed enzyme inactivation (modification) in biological systems. We report here that activated neutrophils and HL-60 cells inactivate exogenous GS and GS in E. coli. Activated neutrophils also inactivate their endogenous enzymes and generate DNPH- reactive material. The oxidative modification of proteins by activated neutrophils may represent a possible mechanism of their bactericidal action as well as a mechanism by which these cells mediate host cell damage during chronic inflammation. Futhermore, several enzymes which we have found to be inactivated by the MFO systems have been found by others to accumulate in an inactive or less active forms during aging. We have also observed an increase of DNPH-reactive material in cultured fibroblasts from normal individuals over an age range of 60 years. In addition, DNPH-reactivity is markedly increased in fibroblasts from Wener's and progeria patients compared to age-sex matched controls. These are genetic diseases associated with accelerated aging. These results suggest that oxidatively modified proteins may accumulate in cells as a function of normal or abnormal aging.